Multilayer interference films are known. In such films, a multitude of individual layers are arranged in a repeating sequence, where the smallest repeating arrangement of layers is referred to as an optical repeat unit, sometimes also referred to as a unit cell. Adjacent individual layers have unequal refractive indices for at least one polarization state of light. The individual layers also have optical thicknesses—defined as the physical thickness multiplied by the refractive index of the individual layer—of less than a design wavelength λ0 such that constructive or destructive interference for light components reflected at the interfaces between individual layers can occur to produce the desired overall reflectivity at λ0. (Since a beam of light traveling through a material experiences a refractive index that can change with the polarization state, direction of travel, and wavelength of the light beam, the “effective refractive index” of the material, which takes these factors into account, can be used in this calculation.) In the simplest case, referred to as a quarter-wave stack, the prior art film comprises alternating layers of a relatively high refractive index material (“H”) and a relatively low refractive index material (“L”), each of which have an optical thickness of λ0/4. Each optical repeat unit of such a stack consists essentially of just two adjacent individual layers, one H and one L, and has an overall optical thickness of one-half of the design wavelength.
Such a stack, however, not only reflects light at the design wavelength, but also at integer fractions of the design wavelength that are referred to herein as higher order reflection wavelengths shown generically in FIG. 1. In that figure, which plots in a simplified fashion normal incidence reflectance of a generalized optical stack versus wavelength on linear scales, a first order reflectance band 100 is seen at the design wavelength λ0, a second order reflection peak is seen at λ0/2, a third order peak is seen at λ0/3, and a fourth order peak is seen at λ0/4. Still higher orders, of course, also exist but are not shown. The higher order reflections, beginning with the second order, are shown generally at 110. A true quarter-wave stack has no even-order reflectance bands (λ0/2, λ0/4, λ0/6, etc.) due to symmetry, but does have odd-order reflectance bands. If the H and L layer within a two-layer optical repeat unit have unequal optical thicknesses, the even-order reflectance bands will be nonzero.
The peak reflectance and the spectral width of the first order reflectance band 100 depends on the refractive indices nH, nL of the H,L layers respectively at the design wavelength (and thus also on the refractive index difference Δn=nH−nL), and on the total number of optical repeat units in the stack. Furthermore, it is known to introduce a thickness gradient such that the optical thickness of the optical repeat units changes along a thickness axis of the stack, in order to expand the spectral width of the first order reflectance band 100. The reflective power (determined by peak reflectance and bandwidth) of the higher order bands generally decreases with increasing order number.
The higher order reflectance bands can be undesirable in some applications. For example, if a visibly transparent infrared-reflecting film is desired for solar control in vehicle or architectural window applications, such that λ0 is greater than about 800 nm, one or more higher order reflectance bands can appear in the visible region and impart an undesirable color that changes with viewing direction.
A number of techniques for suppressing at least some of the higher order reflectance bands are known.
In one known approach, the so-called “f-ratio” of the quarterwave stack is controlled to a value different than 50% by making one of the optical repeat unit component layers (H or L) optically thicker than the other (L or H, respectively) throughout the stack. Although this approach can suppress some higher order reflectance bands, it cannot suppress the second, third, and fourth orders simultaneously and is thus of limited applicability.
The second, third, and fourth orders are collectively significant because it is often desirable to reflect light in a wide first-order band extending from just beyond the visible (i.e., starting between about 700 and 800 nanometers) to about 2000 nanometers. Reflection bands beyond 4th order will generally fall in the UV portion of the spectrum and thus not present any coloration problem in the human visible spectrum (about 400 to 700 nanometers). Although a 5th order reflection for a 1st order band at 2000 nanometers will appear at 400 nanometers, such reflections are usually very weak and, being disposed at the edge of the visible where the sensitivity of the human eye is poor, generally go unnoticed.
In another known approach, the optical thickness sequence of the layers is modified such that the number of individual layers in each optical repeat unit is increased from two to six. See U.S. Pat. No. 5,360,659 (Arends et al.). The six layers still alternate between the H and L component materials, but are arranged in relative optical thicknesses of 7:1:1:7:1:1. Such a structure suppresses second, third, and fourth order reflections.
In yet another known approach, a third optical material (referred to herein as “M”) is introduced having a particular refractive index intermediate that of H and L. See U.S. Pat. No. 5,103,337 (Schrenk et al.). See also U.S. Pat. No. 3,247,392 (Thelen). Further, the individual layers are arranged in each optical repeat unit in the order HMLM with relative optical thicknesses of ⅓:⅙:⅓:⅙ respectively, and the refractive indices are chosen to have the relationshipnM=√{square root over (nHnL)}  Eq. (1)
This approach also suppresses second, third, and fourth order reflections.
There is a continuing need for still more multilayer film constructions to be made available to the optical film designer, including constructions that can suppress the higher order reflections.